1. Field of the Invention
The present invention relates to a polymer material which has excellent fire-retardancy, and emits less corrosive gas and less smoke on burning.
2. Description of the Related Art
Polymer materials are desired to be fire-retardant to prevent fire accident or fire spreading in use for insulating materials such as electric wires and cables; sheath materials; enclosures and internal parts of electric, electronic, and office automation apparatuses; interior materials of vehicles; and building materials, and so forth. Many polymer materials for such uses are enforced to be fire retardant by legislation. For fire retardancy of polymer materials, known fire-retardant additives include halogen type fire-retardant additives, magnesium hydroxide, aluminum hydroxide, red phosphorus, and phosphorus compounds. These fire-retardant materials, however, are not perfect, and have disadvantages below.
The halogen type fire-retardant additives, which give a high level of fire retardancy (for example, UL-94V-0, V-1, or V-2) with a small amount of addition, generate soot or smoke in a larger amount on burning. Further, the halogen type fire-retardant additives emit more or less acidic substances such as a hydrogen halide by heat of processing or at the time of fire accident, which would cause corrosion of the machines for resin processing, or produce adverse effects on human health or apparatuses in the vicinity of a fire site.
Metal hydroxides as the fire retardant, such as magnesium hydroxide and aluminum hydroxide, are required to be added to the resin in a larger amount, although they do not emit smoke or corrosive gas. The addition thereof in a larger amount will impair the mechanical strength, light-weight, and other characteristics of the polymer.
The phosphorus type fire-retardant additives such as red phosphorus and phosphoric acid esters are effective in a small amount for polyamides, polyesters, polyphenylene oxides and other engineering plastics. However, they produce less effect of fire retardancy for general purpose polymers such as polyolefins and polystyrenes.
Therefore, a fire-retardant additive is demanded which contains no halogen, emits less smoke and less corrosive gas, and is effective in a smaller amount of addition. Promising techniques there for have been disclosed in which heat-expandable graphite and a synergist are used in combination.
Japanese Patent Laid-Open Publication 6-73251, for example, discloses fire-retardation of polystyrene by addition of a small amount of red phosphorus and heat-expandable graphite in combination. The system of a combination of the heat-expandable graphite and red phosphorus is required to be improved more in smoke retardation although it gives high fire retardancy. Therefore, a synergist for the heat-expandable graphite is sought which achieves effective retardation of smoking.